The Diabetes and Womens Health Study, based on a retrospective cohort design, aims to understand pathways and determinants underlying the progression from gestational diabetes (GDM) to type 2 diabetes (T2DM) and complications . GDM is a common pregnancy complication. Women who develop impaired glucose tolerance and/or GDM in pregnancy are at substantially increased risk for T2DM in the years following pregnancy. Determinants underlying the transition from GDM to T2DM are not well studied and available studies are limited. There is limited information about the genetic and environmental factors that impact this transition in particular. Further, there is limited research aimed at following women with GDM long term through later adulthood for the development of T2DM and co-morbidities. These critical data gaps serve as the impetus for this study with the overall goal of investigating genetic factors and their interactions with risk factors amenable to clinical or public health intervention in relation to the transition of GDM to T2DM. Data collection for this study will build upon two large existing cohorts: the Nurses Health Study II (NHS-II) and the Danish National Birth Cohort (DNBC). In the present study, we will enroll approximately 4,500 women with a history of GDM who were members of either the NHS II or DNBC. After enrollment, we will follow participants for an additional four years to collect updated information on major clinical and environmental factors including, but not limited to, diet, physical activity and anthropometric information that may predict T2DM risk; to collect timed biospecimens including blood, urine and toenails to measure genetic and biochemical markers implicated in glucose metabolism. The study made outstanding progress last year despite the complexities of contractual and logistics arrangements for an international study and IT compliance requirements for federal research. Effective Sept 03, 2013, 3505 women have been enrolled with ongoing data collection. A web-based data management system has been established to monitor and report the study progress daily and to track biospecimen collection, processing, storage and shipment, allowing the study team to have 24/7 access to the study and data. The lead statistical investigator, Dr. Aiyi Liu, is monitoring the quality of the data, which is reported to be excellent. Further study details are available at the Studys website (www.dwhstudy.org). One unique advantage of the retrospective cohort design is the availability of historical data which allows investigators to address relevant research questions and publish findings while pursuing new data collection prospectively. Comprehensive historical datasets with data longitudinally collected over the past 15-20 years from both the NHSII study and DNBC among more than 7500 individuals (with more than 30,000 variables available from each individual) were delivered to the DWH studys data coordinator center (DCC) and transferred to NICHD in March, 2012. Based on the longitudinal historical datasets, more than 20 analyses and manuscripts have been planned and some of the analyses are underway.